BTW:
It is referred to as the internet guest account but that isn't the actual username. The username is different for every machine. It usually starts off with "IUSR_" and then your machine name. "Internet Guest Account" is always the account's full name as labeled by IIS when it is installed.
Regardless,
If an account isn't listed you have to add it.
Click (add-advanced-find now) and it will list off all the user accounts on the machine
You can also click (add-advanced) and simply type in the account name or part of it.
Some more tips:
If on a local machine you always just give the "everyone" account full control which is pretty much going to make anything work.
To see all the user names and groups on the system you can also go to computer management in your server's administrative tools and view all of the accounts and groups there under "Local Users and Groups
Now, if you are using ASP.NET you need to give permissions to an account called ASPNET. It wont show up in the user list. You'll need to click "add" and then type in "ASPNET"
Regarding hosting companies..
Now.. obviously if you are hosting on someone else's server you may not be able to set permissions like this.
Ultimately, if you are hosting somewhere and ASP and Database connectivity is part of your hosting plan. It is the hosting company's responsibility to set these permissions for you when asked or to give you a special interface to do so on your own. If they are not helping you do this then maybe it is time to get a hosting company that is serious about your ASP Hosting Needs.
Also... JUST TO BE PERFECTLY CLEAR
The permissions we are talking about cannot be set via FTP or FrontPage access to your web site.
These permissions usually must be set like shown above or via a special interface meant to set the permissions correctly. For all you people out there messing with the permissions you see in FTP and FrontPage.. you are wasting your time and possibly creating problems in your web.
(There is however is a nice little trick you can sometimes do regarding data connection and FrontPage.)
All ASP scripts that communicate with an Access Database, Upload Pictures, Modify Text files.... are going to need these permissions set in some way or another. We have no control over that fact.